Simple helical antennas appear to have been investigated thoroughly by many persons skilled in the antenna art and are currently used in many different applications. The helical antenna represents a transition between linear-element antennas and loop antennas.
The Antenna Handbook, edited by Y. T. Lo and S. W. Lee, Van Nostrand Reinhold Company, New York, N.Y., 1988, describes helical circularly polarized antennas in FIG. 6 at page 3-31, pages 27-15 through 27-17 and in FIGS. 14 through 19. Single helices and multiple (bifilar) helices for single antennas are illustrated.
A detailed presentation of the basic concepts and analysis of the helical antenna is supplied in Antennas, Second Edition, by J. D. Kraus, McGraw-Hill, New York, N.Y., 1988, in Chapter 7. Satellite-borne arrays of side-by-side helices are shown. Kraus also depicts a number of antenna configurations including a driven helix having a parasitic helix of about the same diameter wound over it for producing increased gain. He also describes end-to-end helices.
In The Antenna Engineering Handbook, R. C. Johnson, Editor, Third Edition, McGraw-Hill, New York, N.Y., 1993, Chapter 13, King et al. present a detailed description of the performance of helical antennas. King et al. describe single or multiple conductors wound into a helical shape. They describe the axial, normal and higher order modes of operation. Helices with uniform diameter, non-uniform diameter and tapering diameters are discussed. The latter types of helices exhibit greater broad-band frequency response and better circular polarization over a wide band than does the helix with uniform diameter. King et al. discuss short axial mode helices as single antenna elements and in side-by-side arrays. They describe helical windings of circular wire and of ribbon-like flat windings on dielectric material.
As discussed by Kraus and King et al., the helical antenna has different modes of energy propagation which are controlled by its geometry. The most common are axial mode and normal mode. Axial mode, used most widely, provides maximum radiation along the helix axis. This mode occurs when the helix circumference is of the order of one wavelength of the propagated energy. The normal mode occurs when the helix diameter is small with respect to one wavelength and yields radiation generally directed broadside, that is, 90 degrees from the helix axis. Such helices are commonly used in flexible antennas for hand held transceivers and provide shorter antennas.
When a satellite communicates with Earth stations in very high frequency (VHF) and ultra high frequency (UHF) bands, two different helices having different geometry are generally required. One helix operates at VHF and the other smaller helix operates at UHF. The two helices are usually mounted separately, either linearly end-to-end, or side-by-side. However, there are disadvantages to each of these methods of mounting.
End-to-end mounting produces a very long antenna which requires a large stowage space or folding the antenna. Deployment on orbit becomes more difficult when the antenna is folded for stowage. Side-by-side mounting often results in asymmetric coverage by the two radiated beams. If an array of helical elements is needed for increased gain, this configuration can become too unwieldy to stow and deploy because of the volume it occupies.
It would be a significant commercial advantage for a multi-band antenna to be constructed with the higher frequency helix mounted concentrically within the lower frequency helix. Such a system would offer a much more compact antenna than those currently available. For example, as a result of the reduced length, the antenna could be compressed, "spring-like," for stowage aboard a launch vehicle and easily deployed to fill length on orbit by releasing the restraint on the "spring." Moreover, the helical elements could be sized to produce radiation in a novel conical mode which produces a radiation pattern with significant advantages for satellite-to-Earth station communications.
The inventors believe that persons skilled in the antenna arts have worked with designs that generally teach away from concentric helices. Others have proposed to use concentric helices with different polarization at the same frequency for uplink and downlink. A right hand helix might be used for uplink and a left hand helix for downlink. It is the inventors' understanding that this method proved unsuccessful in simulations.
Originally the inventors believed that mutual electric field coupling between concentric helices was much smaller than generally thought and pursued concentric helices operating at different frequencies. Successful designs were subsequently discovered when the frequency difference between concentric helices was at least two to one.
The development of a multi-band concentric helical antenna would constitute a major technological advance and would satisfy a long felt need in the satellite and telecommunications industries.